One of the challenges in promoting lifestyle physical activity is that it really needs to be lifelong to continue to provide ongoing health benefits. Few studies have objectively measured the age-related decline in physical activity in large samples or considered the pattern of physical activity over time. Studies with multiple time points and comparable objective measures examining the same people are relatively rare, and in this study, the CARDIA cohort in the USA, 962 adults were monitored over a 10-year period. Actigraph accelerator data were obtained in 2005/6 and again 10 years later. The study examined middle-aged and older adults from biracial US populations.

The results looked at absolute changes in physical activity and sedentary time and noted a decline in light intensity physical activity by about 30 minutes a day, a decrease in moderate to vigorous physical activity (MVPA) of around seven minutes a day, and a reciprocal increase in sedentary time of around 38 minutes per day. The analysis also examine these by gender and population group, and declines in physical activity were particularly noteworthy amongst people of an African American background.

These data corroborate self-report declines in physical activity which have been noted in other studies. Objective assessment demonstrates that much of the decline is in light intensity physical activity, pointing to lifelong and transitional increases in sedentary time through the life course. We focus much of our physical activity efforts on moderate and vigorous activity, but perhaps we should also consider light intensity activity which we promote through efforts at increasing incidental everyday activity, but clearly not sufficiently to prevent these quite substantial declines. Since this reflects a decline in total energy expenditure, it could also be one of the contributory factors to middle-aged weight gain, resulting from decreases in light intensity physical activities rather than any major MVPA changes. Thus, trends in the prevalence of MVPA may miss these total energy expenditure changes in a population sample.

Other cohorts exist with multiple and comparable measures of objectively assessed activity, and further reports should be produced that corroborate these findings in other populations, and also re-focus our policy attention more on light intensity activity as part of the total energy expenditure mix.